What is the significance of a split second heart sound (S2) on a pediatric examination?

Splitting of S2 on Pediatric Examination

A split second heart sound (S2) in children is most commonly a normal physiologic finding during inspiration, but fixed splitting (present throughout the respiratory cycle) is pathologic and strongly suggests an atrial septal defect (ASD) requiring echocardiographic evaluation.

Normal vs. Pathologic S2 Splitting

Physiologic (Normal) Splitting

• Respiratory variation is key: S2 splitting that widens during inspiration and narrows or disappears during expiration is normal in children 1
• Results from increased venous return to the right heart during inspiration, delaying pulmonic valve closure relative to aortic valve closure
• Common in healthy children and adolescents
• Does not require further workup if clearly respiratory-dependent

Pathologic Splitting Patterns

Fixed Splitting (most important to recognize):

• S2 remains split throughout both inspiration and expiration 1
• Strongly indicates ASD - the classic physical finding 1, 2
• Occurs because the left-to-right shunt equalizes filling between ventricles regardless of respiratory phase
• Mandates echocardiographic evaluation 1

Wide Fixed Splitting also seen in:

• Pulmonary stenosis 3
• Right bundle branch block 2

Paradoxical Splitting:

• S2 splits during expiration, closes during inspiration
• Suggests severe aortic stenosis or left bundle branch block 2
• In severe AS, indicates hemodynamically significant disease 2

Clinical Approach Algorithm

Step 1: Characterize the Splitting

Listen at the upper left sternal border (pulmonic area) during quiet respiration:

• Does splitting widen with inspiration and narrow/disappear with expiration? → Normal, no further workup
• Does splitting remain constant throughout the respiratory cycle? → Proceed to Step 2

Step 2: Assess Associated Findings

If fixed splitting is present, examine for:

• Systolic ejection murmur at left upper sternal border (increased flow across normal pulmonic valve)
• Right ventricular heave (RV volume overload)
• Diastolic rumble at lower left sternal border (increased flow across tricuspid valve in large shunts)

These findings together = ASD until proven otherwise 1

Step 3: Obtain Echocardiography

Class I indication for echocardiography 4:

• Any fixed splitting of S2
• Loud or abnormal murmur with split S2 in infant or child
• Any abnormal cardiac finding in a neonate

The echocardiogram must evaluate 1:

• Atrial septum from multiple views (subcostal, parasternal, apical)
• Right ventricular size and volume overload
• Pulmonary vein connections
• Shunt direction and magnitude via color Doppler

Critical Pitfalls to Avoid

1. Don't dismiss fixed splitting as "normal variation" - The only reliable physical exam finding that excludes severe AS is a normally split S2, meaning fixed splitting is never normal 2

2. Don't rely solely on murmur intensity - Small to moderate ASDs may have only grade 2/6 murmurs but still cause significant hemodynamic consequences 1

3. Don't delay evaluation in neonates - Fixed splitting with any respiratory distress, cyanosis, or abnormal pulses requires urgent echocardiography 4

4. Recognize that absence of splitting doesn't exclude disease - A single S2 can indicate severe AS (A2 soft or absent) or severe pulmonary hypertension (P2 markedly delayed) 2

Quantitative Assessment

Recent research demonstrates that S2 splitting time ≥31.75 ms at end-expiration has 90% sensitivity and 66% specificity for detecting ASD in children 5. While this requires specialized equipment, it validates that prolonged splitting is pathologic.

When Echocardiography Can Be Deferred

Class III (not indicated) 4:

• Asymptomatic heart murmur with clearly respiratory-dependent S2 splitting in a child or adolescent, identified by an experienced observer as physiologic
• No other abnormal cardiac findings
• Normal growth and development

Bottom line: In pediatric examination, assess S2 splitting during quiet respiration. Respiratory variation = reassuring. Fixed splitting = ASD until proven otherwise by echocardiography. The physical examination finding of fixed S2 splitting has stood the test of time as a reliable indicator of hemodynamically significant ASD requiring intervention 1.